Download CRUCIFER SEED PRODUCTION

BRASSICA SEED PRODUCTION
An organic seed production manual for seed growers
in the Mid-Atlantic and Southern U.S.
Copyright © 2005 by Jeffrey H. McCormack, Ph.D.
Some rights reserved. See page 24 for distribution and licensing information.
For updates visit www.savingourseeds.org
For comments or suggestions contact: [email protected]
For distribution information please contact:
Cricket Rakita
Carolina Farm Stewardship Association
www.carolinafarmstewards.org
www.savingourseed.org
P.O. Box 448, Pittsboro, NC 27312
(919) 542-2402
or
Jeff McCormack
Garden Medicinals and Culinaries
www.gardenmedicinals.com
www.savingourseeds.org
P.O. Box 320, Earlysville, VA 22936
(434) 964-9113
Funding for this project was provided by the Southern Regional SARE (Sustainable Agriculture Research and
Education) branch of USDA-CREES (Cooperative State Research, Education, and Extension Service).
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TABLE OF CONTENTS
Botanical classification ..............................................................................................
Climate and soil requirements ................................................................................
Plant characteristics ...................................................................................................
Isolation distances ......................................................................................................
Minimum population size .........................................................................................
Techniques for avoiding inbreeding depression .................................................
Seed production methods..........................................................................................
Cabbage .........................................................................................................................
Cauliflower and heading broccoli ...............................................................
Sprouting broccoli ...........................................................................................
Collards and kale .............................................................................................
Brussels sprouts and kohlrabi .....................................................................
Insect and disease management .............................................................................
Insect pests ..................................................................................................................
Diseases ........................................................................................................................
Selected bibliography and literature cited ............................................................
Credits ...........................................................................................................................
Distribution and licensing information .................................................................
SCOPE OF THIS MANUAL
The scope of this manual is devoted mainly to the major cole crops: broccoli, cabbage, and
cauliflower, and the minor cole crops: Brussels sprouts, kale, and collards. Normally these are all
biennials, growing vegetatively the first year and flowering the next.
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Brassica Seed Production
BRASSICA SEED PRODUCTION
BOTANICAL CLASSIFICATION
Botanical classification of commonly cultivated species of Brassica):
Family: Brassicaceae (mustard family), formerly called the Cruciferae
Common name
Broccoli
Broccoli (sprouting type)
Brussels sprouts
Cabbage
Cauliflower
Chinese kale
Collards
Kale
Kohlrabi
Mustard greens
Broccoli raab
Pak-choi
Pe-tsai
Spinach mustard
Turnip
Rutabaga
Radish
Sea kale
Siberian kale
Horseradish
Cress, upland
Cress, garden
Cress, watercress
Scientific name
Brassica oleraceae
Brassica oleraceae
Brassica oleraceae
Brassica oleraceae
Brassica oleraceae
Brassica oleraceae
Brassica oleraceae
Brassica oleraceae
Brassica oleraceae
Brassica juncea
Brassica rapa
Brassica rapa
Brassica rapa
Brassica rapa
Brassica rapa
Brassica napus
Raphanus sativus
Crambe maritima
Brassica napus
Amoracia rusticana
Barbarea verna
Lepidium sativum
Nasturtium officinale
Horticultural Group
Botrytis
Italica
Gemmifera
Capitata
Botrytis
Alboglabra
Acephala
Acephala
Gongylodes
Ruvo
Chinensis
Pekinensis
Perviridis
Rapifera
Napobrassica
Pabularia
More members of this plant family are grown as vegetables than any other botanical family.
Though there is quite a bit of variability in the vegetative appearance at the mature, edible stage, the
young seedlings are very difficult to tell apart. Brassicaceae are easily recognized by the cross-shaped,
yellow flowers that give the family their former name, the Cruciferae (often used interchangeably with
Brassicaceae). The leaves of some members of this family also have a cross-like (cruciform) shape at
the distal end of the leaf.
CLIMATE AND SOIL REQUIREMENTS
The ancestors of cabbage and its relatives are native to the coastal regions of southern Europe,
from Greece to Denmark, especially along the eastern Mediterranean Sea. The cultivated brassicas
were brought to North America in the 16th and 17th centuries and were commonly cultivated by the
colonists by the 1700’s.
Because of their Mediterranean origin, brassicas are best grown in cool climates with relatively
high humidity. The optimum temperature for growth is between 50 and 77oF (10 and 25oC).
Temperatures above 80oF (27oC) slow or arrest growth. Ideal growing regions are coastal areas where
the climate is cool with moderate to heavy rainfall during the vegetative stage of growth followed by a
non-rainy period during the period of seed harvest.
Of the brassicas, cabbage is the most commonly cultivated, ranking fourth in total vegetable
production in the United States. The most well known regions for cabbage production for market are
upstate New York, Wisconsin, and Ohio where cole crops are started under glass and set out in the
field during the summer. Georgia, Florida, and Texas are the primary cabbage production areas for
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market during the winter months. Plants are seeded in late summer or early fall and then grown and
matured over the winter. As the temperatures rise in the spring, the plants flower and go to seed.
The primary region of cabbage seed production is in the Skagit Valley of the Puget Sound in
Washington State. This region has the most favorable climate, excellent soil, and an abundance of
bees and syrphid flies for efficient pollination.
Seed production in the Mid-Atlantic and South:
The Mid-Atlantic and South can’t match Skagit Valley, but brassica seed can be produced
successfully in many of parts of this region. Two areas offer the best possibilities, the coastal regions
for crops seeded in late summer and early fall, and the mountain regions for heat-sensitive brassicas
seeded in late winter or early spring.
In the coastal regions, the winter temperature is mild enough to allow brassicas to grow slowly or
to winter over in the field without added protection from subfreezing temperatures. In the warmer
coastal areas crops must have at least four to six weeks exposure to cool temperatures to induce
flowering. This is discussed in more detail in the section on vernalization and flowering. The Virginia
Truck Experimental Station (VATES) has traditionally bred and produced a lot of brassica seed in
coastal Virginia. Many of the cultivars bred at the station are still available today.
The mountain regions offer a cooler climate in the summer for vegetative growth of heatintolerant brassicas, but for seed production, the over-wintered plants need to be protected from subfreezing temperatures. Much of the Blue Ridge Mountains and southern Appalachians offer good
growing areas, especially the higher mountain valleys. Some parts of these ranges are cool and with
ample rainfall and some of the higher valleys offer good growing conditions for brassicas. When
plants reach maturity in the fall, they must be harvested with roots intact and stored in outdoor pits
or a root cellar, until plants can be transplanted out in late winter or early spring. Some brassicas can
be left to over-winter in the field provided they are heavily mulched.
Brassicas are hardy to light freezes. Many members of this family are able to tolerate winter
temperatures as low as 15oF (-9oC) for several days especially if hardened off. A few varieties are able
to tolerate brief exposure to cold temperatures as low as 5oF (-15oC). (Brett Grohsgal has bred a
number of brassica cultivars, including Chinese greens and collards, which he claims can take -10oF
(-23oC) temperatures with thirty mile per hour winds for prolonged periods.) At least two varieties of
collards are able to withstand a temperature of 0oF (-18oC) for a brief time. There are large variety
differences in cold susceptibility. Cold tolerance is measured not so much on the lowest temperature
exposure, but the longest sustained low temperature exposure. Other factors have a bearing on cold
tolerance, for example, wind, age of the plant, and amount of nitrogen in the soil, especially as the
plants are becoming established. Too much nitrogen in the soil can lead to softening of plant tissues
and increased susceptibility to freezing and tissue damage.
Members of the Brassicaceae are shallow-rooted and do best in well-drained, fertile soils rich in
organic matter. Early season varieties require higher fertility levels than do late-season varieties. The
presence of rich organic matter is especially important, not only for maintaining high fertility, but also
for proper soil aeration and water-holding capacity. Brassicas do not tolerate waterlogged soil,
drought, or poor soil. A steady supply of moisture is necessary through the growing season;
otherwise supplemental irrigation may be necessary.
PLANT CHARACTERISTICS
General:
Cole crops are succulent, large-leaved, generally low-growing plants that reach a height of one or
two feet before flowering. Upon flowering they reach a height of two to seven feet. Most are biennial,
though most cauliflowers and some broccolis are annual.
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Roots:
The roots of cole crops tend to be fibrous, finely branched, and widely spread. The main taproot
and laterals typically penetrate the soil at an oblique angle, forming a cone-like mass that is mostly in
the upper foot of soil with the narrower (apex) part of the cone reaching a depth of two or more feet
depending on the type and tilth of the soil. A well-grown cabbage growing in good soil may have a
root spread of three feet. A cabbage that has been uprooted in the fall and transplanted in the spring
will develop a very dense root mass. Because brassicas are heavy feeders, they should be well
supplied with nutrients and water during the growing season to help assimilate those nutrients.
Vernalization and flowering:
Length of photoperiod has no significant effect on induction of flowering, but most brassicas
require a period of cold exposure before flowering can occur. This cold requirement is called
vernalization. Typically, biennial brassicas must be over-wintered for vernalization to occur.
Vernalization of the seed, seedling, or young plant is not sufficient to cause flowering because there
must be sufficient nutrient reserves to support flowering. When the plant is in the seedling stage it is
still physiologically juvenile and not sensitive to low temperature exposure, but as the seedling
increases in size, it becomes more sensitive to vernalization. In cabbage, the stem of the seedling has
to be at least ¼” (5 mm) or larger before flowering can be induced. Vernalization requires exposure to
a temperature of 39 to 50oF (4 to 10oC) for at least four to six weeks, depending on the duration of the
exposure and the absolute temperature. The most effective temperature is 45oF (5oC). It is only the
meristem (terminal growing point or bud) that is physiologically sensitive. During cold exposure,
cellular processes are set in action that convert vegetative meristem to reproductive meristem. The
longer the plant is vernalized, the sooner the plant will bolt when temperatures rise. Longer
vernalization also produces more flowers, which can result in greater seed yield per plant. If the
temperature does not fall below 59oF (15oC) plants will usually remain vegetative. For cabbage,
formation of a head is not necessary for vernalization.
Flowers:
Brassica flowers are borne in racemes (rows of simple flowers arranged on an elongated axis) that
grow from the main stem and lateral branches. The flowers have four bright-yellow petals (½ to 1”
long) that are broad on the distal end, and appear in the form of a cross, hence the former family
name Cruciferae (cross bearing). There are six stamens, all of which face the style, but two of the
stamens are usually shorter and lean away from the style: the other four stamens are longer than the
style. The single pistil contains a two-chambered ovary that contains many ovules. The pistil
develops into a long, slender silique (pod) which projects upward at an angle.
Pollination and pollinators:
Cole crops require cross-pollination and are highly attractive to bees. The open architecture of
the flower allows access to many kinds of insects. The principal and most efficient pollinator is the
honeybee: it typically pollinates 85 to 100% of the flowers in large commercial plantings. In smaller
plantings, especially those plantings of small-scale seed growers, wild bees and flies (blowflies in
particular) are important pollinators. In fact, breeders occasionally use blowflies inside pollination
cages to cross-pollinate plants when only a few plants are involved.
Because cole crops grow best in cooler areas, the plants may bloom at low temperatures, often
below 55oF (13oC) which is the minimum temperature for honeybee flight. Several species of wild
bees, if abundant enough, can pollinate cole crops at low temperatures. In fact, wild bees can be more
efficient than honeybees as pollinators of cabbage, even above the minimum temperature for
honeybee flight. Their role is especially relevant in light of the fact that in recent years honeybee
populations have been decimated by the accidental introduction of parasitic mites.
To attract wild bees to small-scale seed production plots, it is important to grow seed plants
within a biologically diverse community. Such a community should include both annual and perennial
flowers that serve as alternate pollen and nectar sources throughout the year when the cole crops are
not available as a food supply. The only sustainable ways to maintain a population of wild bees is to
use organic growing methods, plant perennial flowers that offer alternate pollen sources, and to
model the seed-growing environment (as far as practical) to mimic natural communities. Although
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any form of agriculture is inherently unnatural, the more biodiversity in the seed growing area, the
more successfully plants will be pollinated. In my experience, one of the most challenging aspects of
producing cole crops for seed is ensuring adequate pollination, especially at low temperatures.
Adequate pollination is not only important for overall seed yield, but also is important in producing
larger, more vigorous seed.
The flowers of cole crops open during the early hours of daylight. A few hours later, the anthers
dehisce exposing their pollen. Because of this time differential, cross-pollination is obligatory.
Though pollen is not available to pollinators in early morning, they are drawn to the nectar. Most
varieties have two nectaries located between the ovary and the bases of the two short stamens.
During the three days that a flower is open, the nectaries produce about one-third of a cubic
centimeter of nectar. The stigma of the flower is receptive for about five days prior to anthesis
(pollen release) and up to four days after anthesis. Because the stigma is receptive before the flower
opens it is possible to artificially cross-pollinate the flowers by opening the flower bud and applying
pollen. This technique is called bud-pollination and is used by breeders to produce hybrid seed.
Though too labor intensive to use on a wholesale scale, bud pollination is a useful method for smallscale seed growers who want to work on variety improvement. Another method of producing hybrid
seed is the use of male-sterile breeding lines.
Fertilization and incompatibility:
Most varieties of cole crops are self-sterile. Cross-pollination between flowers on the same plant
does not result in pollination because of one or more biological incompatibility mechanisms. Most
brassica species are predominately self-incompatible, but it is a matter of degree, and the
incompatibility varies with the species, the variety, the age of the plant, and age of the flower. One
mechanism of incompatibility is poor germination of the pollen on the stigma of the same flower
(Nieuwhof, 1969). Another possible mechanism is slow growth rate of the pollen tube in selfed plants
relative to pollen originating from other plants. Regardless of the mechanism, incompatibility is most
strongly expressed in freshly opened flowers, thus reinforcing the reproductive strategy of
outcrossing. When self-pollination does occur, it tends to be late in the life span of the flower (as well
as the life span of the plant). There may be times however, when pollinators are scarce, and as a failsafe mechanism, selfing is allowed by the normal developmental process. This helps to ensure the
perpetuation of the species when pollinators are scarce. On the other hand, results of self-pollination
tend to be characterized by inbreeding depression, lower seed set, and decreased yield in the
subsequent generation.
Some cole crops, notably certain varieties of cauliflower and broccoli (Brassica oleracea), and
turnip (Brassica rapa) may set seed by selfing to a significant degree. Although these species are both
self- and cross-pollinated, cross-fertilization is favored, resulting in better seed set and more vigor.
In summary, even though self-fertilization can occur in certain brassicas (as part of a fail-safe
evolutionary strategy for survival) the best quality seed, the highest yields, and the most vigorous
plants are the result of outcrossing. Without significant outcrossing, brassicas are subject to
inbreeding depression and poor vigor. For this reason, the seed grower must design planting
arrangements using minimum population sizes that foster a maximum of outcrossing. These issues
are discussed later in more detail.
Fruit (pod) or silique:
Brassica flowers produce a silique, commonly called a pod. Siliques range from 1 to 4 inches in
length depending on the species. A silique differs from a true pod in that unlike a pod, the silique has
a thin membranous partition or septum dividing the fruit lengthwise. As the fruit dehisces (opens
and unfolds) the placenta (membranous partition) is exposed. As the placenta dries, 10 to 30 seeds
are released.
Seed:
Seeds of the cole crops are small (approximately 1/16 inch in diameter). Seeds of the different
cultivated species are indistinguishable from each other, except for Chinese cabbage and turnip which
have smaller seeds. When first harvested, the seeds are light brown in color, but as they age, they
gradually change to darker shades of brown, and in some cases to a dark bluish brown. Seed yield
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varies according to species, variety, growing conditions, and pollination efficiency. In cabbage, the
typical seed yield is 12 to 20 seeds per pod. A large, well grown, efficiently pollinated cabbage will
yield up to one half-pound of seed per plant. This figure should not be used for planning purposes
because pollination and growing conditions are rarely optimum. When estimating yield under normal
conditions with average pollination and seed set, a more conservative yield of one-quarter pound of
seed per plant is a more realistic estimate. The average number of seeds per ounce for cabbage,
cauliflower, collard, kale, kohlrabi, Brussels sprouts, Chinese cabbage, and collard ranges between
7,000 and 9,000. Mustard and turnip average 15,000 seeds per ounce, and radish 2,500 seeds per
ounce.
Seed germination:
The statistics on seed germination in relation to temperature are given in the table below:
Type of crop
Minimum (oF)
Optimum Range (oF)
Optimum (oF)
Maximum (oF)
Cabbage
40
45 to 95
85
100
Cauliflower
40
45 to 85
80
100
Radish
40
45 to 90
85
95
Turnip
40
60 to 105
85
105
Note: The optimum temperature for germination is higher than the optimum temperature for
crop growth. These data on soil-temperature conditions for vegetable seed germination were
compiled by J.F. Harrington, Dept. of Vegetable Crops, University of California, Davis.
ISOLATION DISTANCES
Complexities within the brassicas that affect isolation distance determinations
Determination of minimum isolation distances in the production of cole crops is complicated by
the fact that not only will members of the same type of crop cross with each other, but also two
different crops (such as cabbage and cauliflower) will cross with each other if they belong to different
subspecies or horticultural groups (in this example, Brassica oleracea captata x Brassica oleracea
botrytis). An additional layer of confusion arises with Brassica rapa due to the fact that some
members of the B. rapa subspecies may have more than one common name. These issues make seed
production in cole crops much more complicated than other seed crops. Pollinators of the cultivated
brassicas do not make botanical distinctions between the types of flowers they visit. Therefore, for
determining isolation distances, it is essential that the seed grower make the necessary botanical
distinctions, because the pollinators will not.
Adding to this complexity is the fact that certain cole crops (for example, Brassica rapa) are still
in taxonomic flux due to differing interpretations by taxonomists. The Chinese (Asiatic) cabbages and
mustards are illustrative of this point. For example, most taxonomists place Chinese cabbages and
mustards into one of three groups:
•
Pekinensis Group (Chinese cabbage, celery cabbage, and pe-tsai)
•
Chinensis Group (non-heading Chinese mustard, celery mustard, and pak-choi)
•
Perviridis Group (spinanch mustard)
The uncertain taxonomic relationships of Brassica rapa remain to be sorted out by molecular
geneticists. For the seed grower, regardless of the taxonomic classification of the subspecies, it is
important to know that all of these will cross with each other (and with turnip and broccoli raab)
because all these subspecies belong to Brassica rapa.
Another confusing group is kale. By reference to the botanical classification table on page 3, it
can be noted that kale (Brassica oleacea) will cross with other members of the same species, such as
cabbage, broccoli, cauliflower, collards, kohlrabi, and Brussels sprouts. However, several kales (for
example, Hanover Salad and Siberian Kale) are Brassica napus which will cross with rutabaga, but not
sea kale (Crambe maritime) or Chinese kale (Brassica oleracea).
Other factors to consider in determining isolation distances are environmental conditions,
planting time, and earliness to flower. Previously it was mentioned that short-season broccolis
planted in early spring might flower and produce seed by late summer. Also Chinese cabbage and
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Chinese mustard which are normally biennials, when planted early in the season and subjected to hot
temperatures, may flower and produce seed in one season.
Finally, throughout the southeast, there are strong stands of wild mustards and turnips that can
cross with domesticated varieties. Care must be taken to cut back, grazed down, or dig up any such
plants near seed plots before flowering.
Given the various complexities of this group, the take home message for the seed grower is that, if
you have any uncertainty about the proper species classification, the easiest solution is to grow only
one cole crop for seed during the growing season.
Recommended isolation distances for brassica seed production
For a detailed understanding of how isolation distances are determined and utilized for various
seed crops, refer to the manual in this series titled: “Isolation Distances: Principles and Practices.”
This manual covers case studies of certain crops, issues related to large-scale versus small-scale
production. It also gives guidelines for adjusting and interpreting isolation distance
recommendations within the context of the grower’s environment, especially conditions related to
organic agriculture.
Therefore, the discussion here is limited to the recommended isolation distances for cole crops.
In the Isolation Distances manual, the isolation distance recommendations for different crops are
based on the intended use of the seed: the purity of seed required is related to the intended use.
Three different isolation distances are given below, depending on the intended uses which are defined
as (1) home-saved seed, (2) seed saved for exchange, and (3) seed grown for commercial use. In this
manual we are focused primarily on the production of commercial grade seed.
The chart below summarizes the minimum recommended isolation distances for cole crops based
on intended use of the seed:
Type of crop
All cole crops
Minimum
for home
use only
600’
Minimum with
barriers
0.25 mi
Minimum
without
barriers
0.50 mi
Comments
Subject to variables below
Note: When brassica seed crops are grown within range of large commercial plantings, the
recommended distances above should be doubled. As a general rule, every doubling of isolation
distance decreases the amount of cross-pollination by a factor of four. Pollination barriers should
consist of flowers (annuals and perennials) and/or physical barriers (trees, shrubs and dense tall
plantings). When growing organic seed near plots of genetically modified seed, the distances need to
be increased, as discussed in the “Isolation Distances” manual.
Evidence of crossing:
Studies of the amount of crossing in large commercial plantings of brassica seed crops varies
from one-fourth mile to two miles depending on the purity required. For example in one study, 0.6%
crossing was reported between two brassica varieties planted approximately one mile apart (Haig,
1956). This amount of contamination is not significant for production of a commercial crop, but it is
important for a producer of pure seed. Most cross-pollination of brassica crops occurs within 300
feet (Gill and Vear, 1958).
Cross-pollination: an example of the effect of doubling the isolation distance:
Because each doubling of the isolation distance decreases the amount of cross-pollination by a
factor of four, increasing the isolation distance from 300 feet to 1200 feet (nearly a quarter of a mile),
causes the likelihood of cross-pollination to decrease by a factor of 12. Now suppose, for the sake of
illustration, that 10% crossing occurs at 300 feet — at 1200 feet, the amount of crossing would drop
to 0.83%. This distance is 120 feet shy of one-fourth mile (or 1320 feet) which is the minimum
recommended isolation distance given in the chart above — but, this does not include the effect of
barrier crops which reduces the crossing even further.
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Suppose the isolation distance is increased from 300 feet (where 10% crossing occurs) to one-half
mile, the amount of crossing then decreases by a factor of at least 20 (equivalent to five doublings of
the isolation distance). In this hypothetical example, the amount of cross-pollination drops then to
0.5%. This percentage is similar to the 0.6% figure cited (Haig, 1956) for two varieties in large
commercial plantings separated by one mile. Note that in mathematical terms, the amount of
crossing follows a geometric curve, and theoretically never approaches zero at large distances (see
Figure 1), but as seed growers, we are interested in the biological significance. In point of fact, most
pollinators do not fly more than ¼th mile, and the amount of cross-pollination does approach zero
within ½ to 1-mile distance.
Figure 1. General relationship between cross-pollination and distance
from pollinator source.
MINIMUM POPULATION SIZE
Brassicas are outbreeding plants and are therefore subject to inbreeding depression, unless the
population grown for seed is of such a size to allow for a healthy amount of outcrossing. All
interbreeding populations contain a pool of genetic variability. Each gene that controls a certain
characteristic may exist in more than one form of the gene. These different forms of a gene are called
alleles. Some alleles are dominant, and others are recessive (meaning that they are over-ridden or
masked by the dominant genes). When a population size is small, more recessive genes may be
expressed than would normally be expressed in a large population. Some of these recessive genes,
when not masked by dominant alleles may have undesirable qualities such as low vigor, undesirable
growth habit, poor insect and disease resistance, poor flowering, low yield, or off-type foliage. For
this reason, a population size has to be large enough to express the full range of variability and
genetic resources. Another reason for the need of a large breeding population is that some
individuals may be genetically incompatible in terms of their breeding relationships.
The job of the seed grower is to keep the population size large enough while narrowly selecting
for desirable traits. The trade off is that the more you select the variety, the more you narrow the
genetic base. For most outbreeding plants such as brassicas, it is important to save seed from a
minimum of 60 to 75 plants, and in most cases at least 125 to 150 plants.
If the variety has been badly mismanaged when you first acquire it, you should start with about
500 plants. Of those, cull out approximately half the population by selecting out those that have low
vigor, undesirable growth habit, and reproductive maladies such as poor flowering and poor seed set.
The second year your selection criteria can be more narrowly focused on flavor and tenderness, and in
the case of root brassicas, keeping quality in storage, and root shape and quality. Another focus is
selecting for insect and disease resistance.
If the variety has been very well managed and you want to select intensively for one or two traits,
the minimum population should be at least 40 plants. Usually this small population size is used only
when developing a sub-line selection for breeding purposes or to be used with other sub-lines for
multi-line selection.
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TECHNIQUES FOR AVOIDING INBREEDING DEPRESSION
There are two basic methods for avoiding inbreeding depression. The first method has already
been described, that is preserving genetic diversity by means of a large population size. The second
method is to grow the plants utilizing a planting arrangement that encourages cross-pollination,
rather than self-pollination.
Large commercial plantings of brassica seed crops are done in large plots, but small-scale growers
are more likely to plant in long single or double rows. When planting brassicas (or other outbreeders)
for seed production, the plantings should be done in blocks rather than rows. The reason may not be
obvious at first, but it becomes more apparent when considering the behavior of pollinators. In a long
row, a bee will spend more time visiting nearest neighbors in a row, perhaps even re-visiting the same
plant, or the bee may then fly off to visit flowers on an unrelated plant. This behavior of the
pollinator foraging within a linear-geometric context can result in patches of inbreeding depression
within the row. Though the seed from the row will likely be pooled, “patches of inbreeding” persist
within the pool (in a statistical sense) and there will be less heterogeneity in the pool as a whole. On
the other hand, if the seed crop is planted within a block, bees will visit a larger part of the
neighborhood because the “nearest neighbor effect” is larger in a block. In other words, each plant in
a block will be surrounded on all four sides and all four corners (which is eight nearest neighbors). In
contrast, a plant in a row has only two nearest neighbors. Thus block plantings can theoretically
result in four times as much cross-pollination. The four-fold increase in cross-pollination in blocks
versus rows is a hypothetical figure because it doesn’t factor in other variables. Nevertheless it serves
to illustrate the importance of using block rather than row plantings for maximizing cross-pollination.
Suppose you plant your collards in a long single row resulting in some inbreeding depression, can
you regain some of the heterogeneity? The answer is yes. You need only to plant the next generation
of plants in a block, preferably using a larger number of plants. If instead, you plant the next
generation in a long row, the amount of inbreeding will be increased, especially if it is a small
population of plants.
What if you don’t have enough space to grow out the minimum number of plants required to
prevent inbreeding depression? The answer to this is to trade time for space, by pooling the growouts from several years. The first year you grow as many plants as you can and save the seed. The
next year, use only the seed from the original seed lot to grow out another seed crop. The seed lots
from both years are then pooled. As long as you have ample stock of your original seed you can do
this for a number of years.
SEED PRODUCTION METHODS
Cabbage (Brassica oleracea)
Culture:
Cabbage is a biennial, and therefore during the first year of growth, it is grown for seed in the
same manner that it is for eating, market, or processing. Early-season cabbages take two months to
develop heads whereas late-season cabbages can take four months. When grown for seed, the
planting time must be adjusted to allow the proper conditions for wintering over. If planted too early
in the season, the heads may split. If planted too late in the season, the heads may be immature. The
heads should be fully mature before the onset of winter, so that the plants can be dug and stored in a
cellar, or protected in some other way. Winter storage is necessary in some parts of the Mid-Atlantic
where temperatures drop below 20oF (-10oC). In USDA zones 8 and 9, mature heads will over-winter
without protection and may continue growing during a mild winter. Such areas include the coastal
area of Virginia, and coastal and inland regions of the Carolinas, and southern states such as Georgia,
Alabama, Missouri, and Louisiana.
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Propagation by seed:
Seeds are sown ¼” deep in cell packs or pots, two or three seeds per pot. After seedlings emerge
the most vigorous seedlings are selected and the remaining seedling or seedlings are cut off at the soil
line. Seeds are started four to six weeks before transplanting. The temperature range for seed
germination is 45 to 95oF (7 to 35oC) and the optimum temperature is 85oF (29oC).
Rouging:
Rouging is done throughout the growing cycle with particular attention to low-vigor plants. Offtypes can be spotted on the basis of leaf shape and color, but some off-types may not be spotted until
the outer leaves are stripped off prior to storage.
Winter protection and storage:
Late season varieties are the best for winter storage. Early season varieties have more succulent
tissue and are more susceptible to freezing injury. Most cabbage varieties, if sufficiently acclimated to
cold will tolerate 20oF (-7oC). Rapid freezing and thawing are apt to cause more damage than gradual
acclimation, due to the formation of ice crystals which compromise membrane integrity. In cabbage,
there are large variety differences in freezing susceptibility, for example, red cabbage is significantly
less cold tolerant than white Savoy cabbage. It is the pith of the stem that is most sensitive to
freezing injury. The plant may survive, but if the pith freezes it will become spongy and then hollow
as it dries. For this reason, if the cabbage is not dug up for winter storage in a cellar, the soil should
be mounded up high enough to protect the pith by covering the stem. In colder regions, the heads
may be protected by adding 12 to 16 inches of straw over the heads. If rodents are a problem, large
wide boards may be placed in inverted V-shape over the heads with wire mesh added to the ends of
the boards. This then is covered with straw. It is better to hill up soil around cabbage than to dig
them and put them in trenches. If the subsoil is not well drained, water may pool in the bottom of the
trench encouraging disease. The partially exposed tops of hilled cabbage can withstand temperatures
of 5 to 10oF (-15 to –12oC). If the plants are exposed to colder temperatures, they should be mulched
with straw until it is safe to uncover the heads.
Storage of cabbage involves substantial risk, so it is important to have the proper conditions.
Cabbages dug for winter storage in root cellars should have firm, solid heads. Loose-wrapped heads
will not keep well. When digging plants, trim the root system to about 12” long and trim off the loose
outer leaves. The entire plant is then placed on slatted shelves or wire mesh leaving several inches
between plants to allow for good air circulation and ease of inspection. Other methods of storing
cabbage include covering the roots with damp sawdust and wrapping the heads in several layers of
newspaper. Cabbages can also be layered in a thick bedding of straw. Unlike root crops, cabbage
plants cannot be stored in bins or piles because they release a lot of water during storage, especially
in the fall and spring when root cellar temperatures are higher. One other method of storing cabbages
is to tie them upside down from rafters. Regardless of the storage method, the storage temperature
and humidity are important for good results. Keep them cool, between 32 to 40oF (0 to 4oC) at high
relative humidity, at least 80%, and preferably 90%. The low temperature discourages disease while
keeping the cabbage dormant. Though high humidity favors disease, it reduces dehydration. The two
major storage diseases of cabbage are gray leaf mold (Botrytis) and leaf spot (Alternaria). The storage
life of cabbages is three to six weeks for short-season types and four to six months for large slowgrowing types. In the Mid-Atlantic region the storage period may average two to three months.
For organic growers, some important points regarding the storage procedure are: (1) carefully
inspect cabbages for disease prior to storage, (2) avoid nicking or bruising the heads while handling,
(3) inspect the stored crop on a regular schedule, removing any plants that show signs of storage
disease before it can spread, (4) use disease-resistant varieties, (5) grow varieties that store well, and
(6) keep soil nitrogen levels from being excessively high, otherwise growth of soft tissue will be
encouraged, which in turn can lead to freezing damage and disease.
Replanting:
Plants that have been stored in a cellar are set out in the spring as soon as the soil can be worked.
Cabbage can withstand moderate subfreezing temperatures, but as a precaution against extreme cold
weather, the plants can be set low in the soil so that the head is either on the soil or slightly above.
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This helps protect the sensitive stem from freezing. Plants are spaced 24 to 36” apart in rows 3 to 5
feet apart. The spacing between rows is based on plant size and whether the rows will be mulched or
cultivated. Early planting and mulching helps ensure adequate moisture in the soil. As temperatures
become more moderate and weeds begin to emerge, it is advisable to apply mulch to retain soil
moisture. Because cabbage is very responsive to nitrogen, a straw mulch containing manure will
provide a stimulus for early and rapid growth. Within a week or two of planting, two cuts are made at
right angles across the top of each head, no more than 2” deep. This allows the growing point of the
seed stalk to emerge through the head. The cuts should not be so deep that they injure the growing
point, but if not cut deep enough it may be necessary to make repeated cuts so that the seed stalk can
grow, emerge, and develop normally.
Staking:
Staking the flower stalks makes it easier to handle the seed crop when it matures. There is no
significant difference in seed yield between plants that are staked and un-staked when the seed is
mechanically harvested. In small-scale operations, staking does make it easier to move along closely
spaced rows, especially if mechanical cultivation (rototilling) is necessary to control weed growth on
un-mulched soil. Staking also makes it easier to hand harvest the seeds because less bending and
movement is involved in grasping the branches of seed. Staking is done by driving 5-foot stakes into
the ground at intervals of about 10 feet. Two lines of heavy twine are strung between the stakes, each
line woven to one side or the other of the flower stalk, much in the same way the Florida weave
system is used to support tomato vines. A thunderstorm with heavy winds can apply a lot of force on
the seed stalks causing them to bend one way or the other, so it may be necessary to run twine at
more than one height to fully support the stalks. Staking should be done not long after the flowers
begin to set seed because as the pods fill out, the stalks start bending in various directions,
sometimes nearly to the ground.
Harvesting:
One of the problems with harvesting brassica seed is that the pods do not ripen all at once, but
rather in the same sequence of flower opening. This means that as pods yellow, dry and mature to a
light brown color, the oldest pods will dehisce first and scatter the seeds. If only harvesting a small
amount of seed (less than a pound) it is sufficient to visit the crop every day or two and rub the
mature dry seedpods between the hands over a large plastic basin, or a wheelbarrow. Though this
method may give slightly higher germination seed, it is too time consuming for the small benefit
involved. A better method for dealing with larger quantities is to walk down the rows with a corn
knife, machete, or long-handled pruning shears and cut off the maturing seed stalks. This is done
when the basal pods on the seed stalks have matured to a yellow-brown color and some of the other
pods are yellow-brown. The stalks can be harvested even earlier provided the basal pods are yellow
and their seeds are brown. The seed itself, when mature, should not split or crush when rubbed
between the hands. When stalks are harvested by this method, pod color on the seed stalks ranges
from yellow-green to yellow to light brown. The yellowish pods will have quite a bit of moisture
because the seeds are still maturing. The harvested stalks are then brought under cover and placed
on plastic tarps to cure and dry. During the curing process, immature seeds continue to grow and
develop until they become fully mature and dry. When arranging the stalks for drying, the stalks
should not be piled too deeply, otherwise mold may develop. In the Mid-Atlantic and South where
high humidity is a problem, it is wise to allow ample air circulation around the stalks, or to use a fan
to help circulate the air. The process of curing and drying may take anywhere from 10 to 14 days or
longer depending on weather conditions. Some growers recommend pulling the whole plant when the
seed stalks are ready to harvest. Although this method may give slightly better seed germination, the
advantage is minimal. The disadvantages are that this takes much more space, increases the
likelihood of mold, and risks contamination of the seed with bits of soil and small seed-sized stones.
Threshing and winnowing:
The method employed for threshing brassica seed stalks depends on the scale of the operation.
•
Small-scale threshing is done by hand and is suitable for small seed lots. If seed stalks have
been gathered and piled on tarps, some seeds will release during the drying process. The
seed stalks are gathered and twisted or wrung back and forth in gloved hands causing the
seeds to be released into a wheelbarrow or over a tarp. Another method involves walking
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back and forth with a twisting motion over the seed stalks which are spread out on a plastic
tarp or a concrete or wooden floor. Though some sources mention jogging or stomping on the
seed pods to thresh the seeds it should be borne in mind that seeds can be damaged by overly
vigorous threshing. Such damage may not always be visible to the eye but will become
manifest when germination tests are conducted. It is better to wear soft-soled shoes and to
walk firmly over the seedpods with a twisting motion of the feet. The object is not to apply
brute force or a heavy grinding, but rather to apply a light, shearing action to force the
seedpods open. Another possibility that I would like to explore is running the seed stalks
through the wringers of an old wringer washing machine. The rollers of such machines are
spring mounted so that the rollers recede in response to the amount of pressure on the
rollers. By varying the tension on the rollers it might be possible to adapt the wringers for
threshing.
•
Large-scale threshing methods are employed when producing more than 75 to 100 lbs. of
brassica seed. Most threshing machines are too expensive to warrant their purchase by smallscale growers; however they are appropriate when used by a cooperative. Most threshing
machines are called combines which consist of a feeding section and a threshing section. The
feeding section typically consists of a reel, divider, cutter bar, and feed mechanism. The
threshing section consists of rubber-coated rollers through which the milled stalks are fed. (or
in another variation, the stalks are fed between rubber-coated cylinders that rub against
concave fixed bars). The old literature on threshing suggests feeding the butt end of the
stalks into the thresher first. By feeding the thickest part of the stalks first, one can tell the
maximum load on the rollers and thus avoid overfeeding. The feeding should be done at a
continuous steady rate, close to the maximum capacity of the thresher. The general rule in
setting the clearance of the rollers is that the distance between the rollers or cylinder and
concave bar be at least 1-½ times the diameter of the seed. The cylinder speed of the combine
should be less than 700 rpm in order to minimize the splitting of dried seed.
Seed cleaning:
Fortunately the round, fairly uniform shape of brassica seed makes them easy to clean. The first
step is to remove large stems and other debris by pouring the rough chaff and seed through a large
mesh screen such as ¼” hardware cloth. The next step is to clean the seed with two seed-cleaning
screens.
There are two basic types of screens used for cleaning round seed: (1) a round-hole top screen,
and (2) a slotted bottom screen. In the case of brassicas, a 1/8” round-hole screen works well as the
top screen. This allows the seed to pass through while screening out the chaff and larger debris. The
bottom screen is used to catch the seed, while allowing the fine chaff and dirt to pass through. The
bottom screen can either be a slotted screen or fine wire mesh. The size isn’t critical; it need only
retain the brassica seed while allowing the chaff to pass through. After screening, brassica seed needs
to be blown free of fine chaff. If an air separator or seed blower isn’t available, a stainless steel bowl
and a hair dryer are sufficient for cleaning small lots of seed. For cleaning larger quantities, the seed
can be poured in front of a box fan. The seed will be caught by a large container or tarp below the
fan, and the chaff will be blown beyond the container or tarp. Cabbage seed may be sized into several
sizes for precision planting.
One method of separating high-quality brassica seed from poor-quality brassica seed and small chaff
is to use a slant-board. Poor-quality brassica seed is often not round and will not roll all the way
down a board inclined at a shallow angle. To build a slant-board, fasten small strips of wood along
the long edge of a 4 x 8-foot sheet of plywood. Raise the 4-foot edge 18” off the ground and then
evenly disperse ¼ lb of seed along the top edge. Gently tap the bottom side of the plywood until the
roundest seeds make it to the bottom. Discard everything remaining on the slant board.
The basic procedures for threshing and winnowing seeds are covered in the companion manual in
this series titled: “Seed Processing and Storage.”
Additional information on seed threshing and cleaning may be found at www.savingourseeds.org,
and www.agronomy.ucdavis.edu/LTRAS/itech/ .
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Seed storage:
Cabbage seed will remain viable for about five years when properly stored at low humidity (less
than 50%). The seed moisture content should not exceed 6% during storage. For detailed information
on this topic, see the companion manual titled: “Seed Processing and Storage.”
Cauliflower and Heading Broccoli (Brassica oleracea var. botrytis)
Climate and soil requirements:
Cauliflower and heading broccoli are predominately biennial crops that do best in cool, humid
climates. The techniques for growing seed are similar to those for cabbage, but the planting time and
growing temperature are more exacting. For this reason, successful production of seed is more
difficult, though this is offset by a higher return received for the seed crop. Cauliflower and broccoli
are more sensitive to heat and freezing injury. For cauliflower, sowing dates must be adjusted so that
both young plants (at the first stage of development), and older plants (during the stage of curd
initiation) are exposed to cool temperatures. Exposure to high temperatures during these critical
periods may have an adverse effect on development and seed production. The optimum temperature
for growth of young cauliflower seedlings is 74oF (23oC), and for more mature plants is 64 to 68oF (17
to 20oC). Temperatures above 84oF (29oC) will slow vegetative growth and delay curd initiation, and if
the curd has already formed, the bracts on the flower stalks may grow and protrude through the head.
Like cabbage, cauliflower can withstand frosts and sub-freezing temperatures provided the curd is
protected by tying up the wrapper leaves over the curd. Cauliflower, depending on the variety, can
survive to 18oF (-7oC) provided that the plant has been slowly acclimated and hardened to endure subfreezing temperatures. Tying up the leaves or handling the plant during sub-freezing temperatures is
not recommended because tissue damage may result. Like cauliflower, the pith of the stem is most
sensitive to freezing injury, as well as young succulent growth.
Annual forms of cauliflower are not as temperature sensitive as biennial types. Seed can be
produced during a single, sufficiently long season, though if the season is not long enough, the plants
may need to be transplanted into a greenhouse. Annual forms do not require vernalization nor is
there a minimum size requirement for curd development. When annual varieties are grown at a
temperature above 68oF (20oC) the heads will be of low quality. Annual varieties of cauliflower
developed in tropical regions such as India will form heads at moderately high temperatures.
Heading broccoli is less demanding than cauliflower. Unlike cauliflower, it is less sensitive to warmer
conditions and more tolerant of lower soil moisture. Like cauliflower it needs favorable growth
conditions for the formation of high quality heads.
Primary seed production regions:
Cauliflower is a specialty, high value crop because of its growing requirements. In the United
States the primary areas of seed production include the middle coastal areas of California, the Puget
Sound in the state of Washington, and historically Long Island, New York.
Seed production in the Mid-Atlantic and South:
Cauliflower and heading broccoli are best grown for seed in mild winter climates. This is
primarily cool coastal areas where the winter is mild enough to cold enough to allow for vernalization,
but not so cold that the plants are subject to freezing. Like cabbage, heading broccoli and cauliflower
can be dug for winter storage in late fall/early winter but the plants remain in good condition for only
six weeks at best. This requirement defines those locations where seed growing can be successful.
Culture:
Many cultivars of cauliflower and heading broccoli are biennial, and therefore during the first year
of growth, they are grown for seed in the same manner as for eating, market, or processing.
Depending on the variety, plants are spaced 2-½ feet to 3 feet apart in rows 4 feet apart. A constant
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supply of water is required to grow these crops well. The soil should neither be waterlogged, nor
allowed to dry out. Organic growers are advised to use surface irrigation (drip hoses) rather than
sprinkling which favors disease. If overhead sprinkling is used, it should be done in the morning so
that the leaves dry off during the day.
Propagation by cuttings:
The cultivated brassicas can be successfully propagated by cuttings. In Japan, seed producers
have used cuttings of broccoli to produce seed. The technique involves sowing the seed in the spring
and harvesting the head two months before a killing frost. The plant is then cut to form a stump 4 to
6” tall. After the side shoots grow to a length of 4”, the shoots are treated with a rooting hormone and
planted in heated soil in nursery beds. The rooted cuttings are planted in the spring, and because
they are vernalized they produce seed. This production method using synthetic growth regulators is
not appropriate for organic growers, but the technique raises some interesting possibilities. Nonsynthetic growth regulators are allowed for organic production. One natural source of rooting
hormone is from “willow water.” Willow water is made by placing cuttings of the willow tree or pussy
willow (Salix spp.) in a container of water for several days. The water is then collected (and frozen for
later use). The collected water contains a high concentration of auxin, a natural growth regulator
called indole acetic acid (IAA) which stimulates root formation. When rooting a cutting, the basal end
of the cutting is cut ¼” below the lowest node, and then placed in a natural substrate. This technique
might be employed successfully by rooting cuttings in a cool greenhouse where the nighttime
temperatures are kept below 50oF (10oC). Though I am not certain whether the technique would work
it might a technique worth investigating.
Developmental phases and abnormalities:
Cauliflower and heading broccoli thrive best when growth is slow, steady and consistent without
environmental extremes or stresses. Cauliflower is especially sensitive to stress during the transition
period between the three major stages of development: the vegetative stage, curd formation stage, and
maturation of the head. The most common stress-related problems are buttoning, no heads
formation, leafy curds, and riciness.
•
Buttoning is characterized by the formation of small tiny curds when the shoot is very small.
The phenomenon is not well understood, but appears to be caused by moisture stress (too
much or too little), transplanting of root-bound plants or old plants, transplant shock, mineral
nutrient stress (primarily low nitrogen), and excessive low temperature during a change in
developmental phase. In other words, buttoning is a stress-related phenomenon, often with
multiple causes that may act singly or together. Heading broccoli is also subject to buttoning,
though to a smaller degree. If buttoning occurs, it is most likely to occur shortly after
transplanting. It can be prevented by making sure that the transplants are young and actively
growing with no checks in growth prior to transplanting. Once the transplants are established
in the field, environmental stresses are much less likely to cause buttoning. Buttoning is
more a problem with early varieties which can be induced to develop curds at an earlier
developmental stage. The basic precautions to avoid buttoning are:
o Don’t expose transplants to freezing temperatures.
o Keep the air temperature above 70oF ( 21oC) and soil temperature above 60oF (16oC)
during the seedling stage.
o Transplant 4 to 5 week old plants. Don’t transplant large, old, or root-bound plants.
o Minimize or avoid transplant shock.
o Keep an eye on the weather forecast and avoid transplanting when temperatures are
below 50oF (10oC) for more than four days.
o Maintain conditions for good steady growth, notably proper water and nutrient levels.
•
No head formation is the result of high temperatures keeping the plants in the vegetative
phase until temperatures fall low enough for curd formation to take place.
•
Leafy curds are the result of improper nitrogen balance or large swings in day or night
temperatures.
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•
Riciness is a condition characterized by a rough or granular appearance of the curd. This can
be caused by low temperatures or improper (often excessive) nitrogen availability after
transplanting. This is less of a problem for organic growers who rely on available soil nitrogen
rather than amendments after transplanting.
•
Blindness is a condition characterized by collapse of the growing point that results in coarse
leaves, no main curd, and small curds that develop from axillary shoots. Blindness can occur
when seedling are exposed to freezing temperatures.
•
Nutrient deficiencies: There are two nutrient deficiencies that affect cauliflower in
particular:
o Boron deficiency: This occurs when the soil pH is high, or soil boron levels are low.
Boron availability to plants declines markedly above soil pH 7.0. At pH 7.5, the
availability of boron decreases by half, and at pH 8.0 the availability decreases to onefourth. Symptoms of boron deficiency include collapsed internal pith resulting in
hollow stems, chlorotic leaf margins in older cauliflower plants, and elongated, dark
green brittle leaves in young plants. Boron deficiency can be prevented by lowering
the pH by addition of organic soil amendments such as composted pine needles, or
oak leaves, or other acidic composted plant material. If soil levels of boron are low,
organic growers may increase boron levels by application of certain non-synthetic or
synthetic soluble boron products (e.g. borax at 20 to 30 lbs. per acre), but growers
must first document the deficiency by testing. Foliar application of boron may be
helpful. Consult your organic certifier before applying regulated boron products.
o Molybdenum deficiency: In severe cases, this causes a condition also known as
“whiptail,” which is due to the failure of the leaf lamina to develop properly along the
midrib of the leaves. Plants with whiptail will be stunted and either won’t produce a
curd or the curd will be leafy and poor quality. In milder cases of molybdenum
deficiency, the leaves become narrower with ruffled, irregular margins. Young
seedlings have cupped leaves with chlorosis around the leaf margins. Molybdenum is
fully available at soil pH 7.0 and above, but availability declines with increasing
acidity. In pH 6.0 soil, molybdenum availability is only half that compared to neutral
soil. Adjustment of soil pH is the best prevention; otherwise consult your organic
certifier.
Rouging:
Rouging is done throughout the growing cycle with particular attention to low-vigor plants. Offtypes can be spotted on the basis of head characters such as size, depth, uniformity, color, and
compactness. Other undesirable characteristics include loose curds, leafy curds, and non-uniformity.
Because cauliflower is an expensive crop to produce, it pays to rogue carefully.
Flowering:
The heads of cauliflower contain a number of non-functional flowers, which is one reason why the
seed yield is lower than cabbage. On the other hand, purple cauliflower has fully functional flowers.
The flowering stalk of cauliflower and heading broccoli is much shorter and more compact than that
of cabbage, usually not more than 24 to 30” tall, compared to cabbage which is typically 48” tall. In
addition, the flowering occurs within a period of 10 to 14 days which is a much shorter flowering
period than cabbage.
Harvesting, threshing and cleaning:
Cauliflower and heading broccoli are processed the same way as cabbage. Cauliflower seed crops
tend to mature later than most cole crops. Seed yields average 150 to 200 (up to 350) pounds per acre
(approximately 4 pounds/1000 square feet) compared to cabbage which averages 400 to 800 pounds
per acre (9 to 18 pounds/1000 square feet).
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Sprouting Broccoli (Brassica oleracea var. italica)
General:
The term sprouting broccoli refers to plants that produce small heads of immature flower buds.
Sprouting broccoli does not produce a central head. White sprouting broccoli is actually a type of
cauliflower.
Climate and soil requirements:
Sprouting broccoli is easier to grow than cauliflower. It is adapted to warmer and less humid
climates, planting times are more flexible, and is not as demanding in its requirements. Though more
tolerant of warmer climates, best growth of broccoli occurs during the cooler months of spring and
autumn. The optimum temperature range for growth is 59 F to 68oF (15 to 20oC). The maximum
temperature for production is 77oF (25oC). Temperatures above 82oF (28oC) will delay initiation of
heading and flowering, and if bud clusters (small heads) have formed the flower stalks will lengthen,
the bud clusters will become looser and will wilt rapidly after harvest. Soil should be kept moist,
otherwise drought will result in the production of smaller heads and more fiber in the stalks. Once
plants approach maturity plants should receive 1 to 1½” of water a week. The sensitivity of broccoli
to freezing injury depends on the developmental stage. A light frost is of little consequence, but the
floral buds and pith within the stalks are sensitive to sub-freezing temperatures. Freezing injury will
be manifested as water-soaked tissue followed by rotting of affected tissue.
Plant characteristics:
Sprouting broccoli typically has a small central head and numerous side heads, which develop
over a period of weeks. Plants are usually about 24” tall, but when the heads of broccoli mature
beyond eating or market stage the small heads expand and the flowering stalks expand to form an
inflorescence similar to that of cabbage.
Culture:
Broccoli, like cabbage requires fertile soil and grows best when well irrigated. Like other cole
crops, the soil should be high in organic matter to increase drainage and water-holding capacity.
When planted for home or market, broccoli is spaced 16 to 24” in the row and 18 to 36” between
rows, but when grown for seed the rows are spaced 30 to 40” apart. In the South, broccoli is often
direct seeded but in the Mid-Atlantic plants are started as transplants after the danger of subfreezing
temperature has passed. Spring-planted broccoli matures a seed crop in the fall, and fall-planted
broccoli which has been over-wintered matures seed the following summer. One of the factors that
differentiates broccoli cultivars is temperature tolerance. Early varieties that mature in 55 to 80 days
tolerate higher temperatures, whereas later varieties that mature in 80 to 105 days are more suited for
fall production. Soil fertility should be managed to encourage moderately rapid growth. Growth that
is too rapid can produce hollow stems. Growth that is too slow due to limited nutrition will result in a
low seed yield. Broccoli is similar to cauliflower in being sensitive to boron deficiency.
Fertilization and incompatibility:
Like other cole crops, broccoli is not normally self-fertile, so consideration should be given to
factors that avoid inbreeding depression as discussed previously.
Rouging:
Aside from rouging out low vigor plants, emphasis should be placed on removing off-types.
Though foliage characteristics should be noted when rouging, the characteristics of the head are the
most important. Desirable characteristics include compact heads with buds of similar type and color
and heads that lack leaves. It is important to know the variety when rouging. For example, some
varieties are known for their dark green or blue green color in contrast to the medium or lighter green
color of other varieties. Other variety specific qualities include bud side and size of number of side
shoots.
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Harvesting and threshing:
Broccoli seed is harvested, cured and threshed in the same manner as cabbage, and because it is
usually grown in a warmer and drier climate than cabbage, the seed cures more rapidly.
Collards and Kale (Brassica oleracea var. acephala)
Climate and soil requirements:
Both collards and kale are biennials that produce seed the second year. Kale and collards are
considered the more “primitive” of the cole crops in that they more closely resemble wild cabbage.
They were grown by the ancient Greeks, and later introduced into Britain and France by the Romans
and Celts. Their climate and soil requirements are similar to cabbage, but they are hardier than
cabbage, tolerating both low and high temperatures better. Collards, in general, are hardier than kale.
For seed production, winter temperatures should be below 50oF (15oC) for at least six weeks for
vernalization to occur. Most adult plants can withstand winter temperatures down to 14oF (10oC)
provided the growing location is well drained. Several varieties can survive down to 5oF (-15oC) (As
noted earlier, Brett Grohsgal has bred varieties that can tolerate lower temperatures). Collards can be
grown where it is too hot to grow cabbage.
Seed production in the Mid-Atlantic and South:
For seed production in the Mid-Atlantic region using the seed-to-seed method, cold-hardy varieties
may be left in the field to over-winter in USDA zones 7b and higher, otherwise zones 8b through 10
for sensitive varieties. Some varieties bred by Brett Grohsgal are winter-hardy to zone 6a. though still
unprotected In areas where winter lows are too low to leave the unprotected plants in the field, the
plants are started in late July, and are grown to only moderate size in the fall (approximately 8 to 12”),
and then transplanted in late fall to pots to be brought into the greenhouse. The plants are then set
out in the early spring. Another alternative is to thoroughly mulch the plants in the late fall, carefully
protecting the central stem and growing tip, during the coldest part of the winter. One other
technique that may be used is to construct an inexpensive hoop house in the field. This can be done
by cutting 100-foot rolls of flexible 1” (outside diameter) water supply pipe into 5 to 7 foot hoops
(depending on height of plants to be protected). The ends of the pipes are inserted over 8 to 12”
pieces of rebar, rigid plastic pipe, or galvanized metal conduit that is ¾” or less (outside diameter).
The hoop house frame is then covered with 6-mil polyethylene. Boards may be used to hold down the
edges of the plastic, but a strong gust of wind may pull the plastic from beneath the boards. For that
reason, u-shaped soil clips are a better solution. These are normally used to secure floating row cover.
Hoops should be spaced no more than 18 to 24” apart, otherwise a heavy wet snow load may bend or
collapse the hoops. Venting the hoop house is important on warm winter days, otherwise inside
temperature and humidity may rise high enough to damage the plants or affect their cold hardiness.
Plant characteristics:
Collards and kale are considered minor brassica crops because there are fewer varieties and the
total amount of acreage devoted to seed production is much lower than for cabbage, cauliflower, and
broccoli. Kale resembles wild cabbage more than most cultivated brassicas. Kale has an open type of
growth with typically curly leaves and no head. Varieties of kale vary considerably in growth habit,
plant size, and leaf color, texture, and thickness. Collards are much less variable. Whereas most
varieties of kale are shorter with a thinner stalk, collards have a thick central stem that terminates in a
loose, cabbage-like foliage. Some varieties are non-heading, whereas others form a loose head or
cluster of leaves at the top of a long stalk.
Flowering and pollination:
Pollination and incompatibility relationships of collards and kale are similar to cabbage. Likewise,
a vernalization period is required in order for flowers to be produced. In some locations in the deep
South (for example, southern Louisiana), collards may not act like a true biennial. There, collards can
be planted any month of the year and will produce seed stalks by early March. The temperature and
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light conditions will cause two-month old plants to flower at the same time as nine-month old plants,
but of course the older plants will produce much higher seed yields.
Rouging:
Rouging for trueness to type is especially important with the minor crops because seed
companies tend not to invest much in their development and maintenance. When rouging collards
and kale special attention is given to the edible part, sweetness, tenderness, and texture. Other
qualities include cold hardiness, insect resistance, and disease resistance.
Harvesting and threshing:
Collards and kale are handled similarly to cabbage. Seed yields are lower than cabbage, ranging
from 300 to 600 pounds per acre (7 to 14 pounds/1000 square feet).
Brussels sprouts (B. oleracea var. gemmifera)
and Kohlrabi (B. oleracea var. gongylodes)
General:
Both Brussels sprouts and kohlrabi are minor crops of relatively recent origin. Brussels sprouts
dates from the late 1500’s in the region of Brussels and Belgium. The ancestor of Brussels sprouts is
thought to have been a type of Savoy cabbage in which the terminal growing point was routinely
removed, resulting in the formation of axillary buds similar to Brussels sprouts. Normally, Brussels
sprouts grow 3 feet tall with a rosette of leaves at the top with compact miniature “heads” which are
large axillary buds that develop along the stalk. Dwarf varieties have shortened internodes. The
sprouts at the top tend to be smaller than those at the base because of apical dominance, hormonal
suppression of growth of the upper buds.
Kohlrabi was also developed in the late 1500’s in northern Europe and became popular in
Germany where it acquired the German name kohlrabi, which means turnip. Botanically it is not a
turnip, but rather a variant of cabbage with a swollen stem. The stem does not elongate and the
leaves are arranged in spiral fashion.
These crops are handled essentially the same as other biennial cole crops using the seed-to-seed
method and allowed to winter over.
In terms of heat tolerance, kohlrabi and Brussels sprouts are more tolerant than cabbage, but less
tolerant than kale and collards. Regarding kohlrabi, early varieties subjected to cold temperatures
below 50oF (10oC) for one week or more after germination are susceptible to early bolting. Slow
bolting varieties require a longer exposure to vernalization temperatures (below 50oF (10oC)), and
there is no correlation between earliness and cold temperature exposure. Kohlrabi and Brussels
sprouts may be over-wintered in the field where temperatures permit, otherwise they may be dug in
late fall and stored in a cellar at 32 to 40oF (0 to 4oC) and high humidity.
Rouging:
Proper rouging is important for the minor cole crops because they may have not been as well
maintained or selected. Both should be rouged at the time of prime market maturity. Kohlrabi should
be selected for uniform size, shape, and color. Brussels sprouts should be rouged for large compact
buds that are firm and uniform.
Harvesting and threshing:
The seeds of kohlrabi and Brussels sprouts are harvested and threshed in the same manner as
other cole crops. Seed yields from kohlrabi average 800 to 1,300 pounds per acre (18 to 30
pounds/1000 square feet). Seed yields from Brussels sprouts average 300 to 600 pounds per acre (7
to 14 pounds/1000 square feet).
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INSECT AND DISEASE MANAGEMENT
As a preface to the sections on insect pests and diseases, it should be noted that the control
treatments recommended are for organic growers. Regulations regarding approved methods may
change from time to time and approval of certain methods of control may depend on the certifying
agency. Check with your organic certifier before using pest and disease control treatments. There is
no substitute for prevention via good crop management. This includes: (1): proper attention to crop
nutrition and growth conditions, (2) crop rotation, (3) sanitation, (4) maintenance of biodiversity to
encourage beneficial organisms (4) scouting to detect problems at an early stage to allow time for
intervention, management and control, and (5) use of spot treatments instead of broadcast treatments
where appropriate. In the material below, mention is made of varieties that have resistance to certain
diseases or insects. Bear in mind that resistance is often not complete, may be subject to change, and
may depend on environmental conditions.
INSECT PESTS
There are a number of insects that feed on cole crops. Pests include cabbage aphids, flea beetles,
beet armyworm, cutworms, and cabbage maggots. The major insect pests for cole crops grown in the
Mid-Atlantic and South are described below.
•
Cabbage looper (Trichoplusia ni): The adult is rarely seen because it is a night-flying moth. It
has mottled gray-brown wings with a sock-shaped silvery spot in the center of each forewing.
The caterpillar stage is found predominately on the underside of the leaf, along leaf margins,
but may be found anywhere on the plant feeding between the leaf veins. Because cabbage
loopers have no middle legs, they loop their bodies as they crawl. When disturbed, larvae may
drop to the base of the plant. Mature caterpillars are 1½” long with two thin white stripes
down the back and two wider pale white stripes along the sides. The ridged, round white eggs
are laid singly on the underside of outer leaves.
•
Diamondback moth (Plutella xylostella): The moth is small and narrow with folded wings.
The female is gray-brown but the male has three yellow diamond-shaped spots where the
wings meet. The yellow-white, football-shaped eggs are laid singly or in small groups on the
underside of the lower leaves. The larvae are small, not much more than ¼” long when
mature. Yellow-green in color, they have a “forked tail” and black head. When disturbed they
often drop or hang from the plant by a thin silk thread. They prefer to feed on new growth
and the buds of young plants. It is important to scout the plants when they are young,
especially because feeding may cause malformations.
•
Imported cabbage worm (Pieris rapae): The adult is a butterfly with white wings and black
spots on the forewing. When the flying adults are seen, it is time to start scouting for damage.
The female chooses seemingly random places to lay an egg, one at a time anywhere on a
single plant and similarly on a number of plants. The eggs have distinct ridges and are white
when laid but turn a dusky yellow when they mature. The mature larvae are 1¼” in length
and are identified by the light yellow stripe down the middle of the back. Frequently they are
found feeding next to the midrib or veins on the underside of the leaf. The caterpillars
pupate on the underside of the leaves. Damage can be extensive on young plants.
•
Cross-striped cabbageworm (Evergestis rimosalis): The yellow-brown moth has zigzag
markings on the wing. It lays light yellow, scale-like eggs in masses of 20 to 30 on the
underside of leaves. This late season caterpillar is bluish-gray with numerous tiny black
stripes running transversely across the back. On each side of the body is a black and yellow
stripe. Mature caterpillars are ¾” long. Because the eggs are laid in clusters, scattered
individual plants may be infested severely. Larvae prefer young terminal growth, often
riddling it with holes. When the larvae are ready to pupate they drop to the soil and form a
cocoon just under the soil surface.
•
Corn earworm or cabbage headworm (Heliocoverpa zea): This is a late season pest often
encouraged by the presence of a corn crop.
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•
Harlequin bug (Murgantia histrionica): The harlequin bug is a major pest of cole crops in
the Mid-Atlantic and South. It can devastate most cole crops if not controlled. Easy to
identify, it is shiny, red, orange, and black, shield-shaped bug, about ¼” to ½” long. It may
produce three or four generations per year, but in the South it may breed year-round. The
distinctive white and black barrel-shaped eggs are laid in double rows on the underside of
leaves.
Control measures for caterpillar pests:
Fortunately, control of caterpillar pests is not difficult. The most important measure is thorough
scouting on a regular basis. Also, a growing environment that encourages biodiversity is important
for supporting predatory and parasitic wasps (such as Cotesia and Diadegma) that prey on
caterpillars. Other than creating a conducive environment for natural control, organic growers can
easily and effectively use Bacillus thuringiensis (also called “B.t.”) provided that the formulation used
does not contain prohibited materials. Trichogramma wasps, which parasitize moth eggs, can also be
purchased. Yet another part of the strategy involves use of resistant varieties. Varieties with reported
resistance to caterpillars include the following:
•
Broccoli: ‘Atlantic’
•
Cabbage: ‘Mammoth Red Rock’, ‘Chieftain Savoy’, and ‘Perfection Drumhead Savoy’
•
Cauliflower: ‘Snowball A’ (some resistance)
•
Collards: ‘Cascade Glaze’ and ‘McCormack’s Green Glaze’
•
Kale: ‘Vates’ and varieties with the green glaze leaf (in development)
•
Mustard: ‘Southern Giant Curled’
Control measures for harlequin bugs:
In small plantings consisting of a few plants, they may be controlled by hand picking. The best
prevention is early scouting and use of insect-resistant varieties. Several varieties of collards and kale
are quite resistant due to the glaze leaf characteristic. Breeding programs should focus on
incorporating the glaze leaf characteristic into other brassicas. Natural pyrethrum or other approved
botanicals may be used provided documentation of lack of alternatives is provided. Varieties with
reported natural resistance include:
•
Broccoli: ‘Atlantic’
•
Cabbage: Copenhagen Market’, ‘Early Jersey Wakefield’, ‘Perfection Drumhead Savoy’, and
‘Stein’s Flat Dutch’
•
Cauliflower: ‘Early Snowball Y’ and ‘Early Snowball X’
•
Collards: ‘Cascade Glaze’ and ‘McCormack’s Glaze’
•
Kale: Varieties with the green glaze leaf (in development)
•
Mustard: ‘Old Fashion’ (some resistance)
DISEASES
Any disease that affects cabbage can also affect other brassicas to a greater or lesser degree.
Prevention plays a key role in controlling diseases, and the most basic steps involve proper
management of insect pests, good soil nutrition and growth management, composting crop residues,
removal, destruction of diseased material through burial, and crop rotation. It is important that cole
crops not be grown in soil that has grown any member of the cabbage family for two to three years
previously. The major diseases that can affect seed growers in the Mid-Atlantic and South are
discussed here.
•
Alternaria leaf spot (Alternaria brassicae and A. brassicicola): This fungus disease is
common to all brassicas. Also called black leaf spot, symptoms appear chiefly as small,
concentric spots with brown-black rings on the lower leaves. Spots may grow to nearly ½”
size during favorable weather. The fungus over-winters in seed and plant residues.
•
Yellows (Fusarium oxysporum f. sp. conglutinans): Also called “cabbage yellows” because it
is most severe on cabbage, the causative organism is a soil fungus. It can persist in the soil
for many years and is favored by high soil temperatures. Symptoms first appear as a
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yellowing of lower leaves, followed by progressive wilting and loss of lower leaves one by one,
and finally plant death. One diagnostic feature is the presence of black vascular tissue.
•
Black rot (Xanthomonas campestris pv. campestris): Black rot is a bacterial disease carried
on the seed and on crop residues. On older plants, symptoms appear initially as v-shaped
yellow areas along the outer leaf edges. As the disease progresses it moves from the leaf
margins to the base of the plant, the yellow areas turning brown and the veins turning black.
Plants are stunted, lower leaves drop, and the diseased areas are invaded by soft rot. Young
seedlings show a different pattern — they yellow and die.
•
Bacterial soft rot (Erwinia and Pseudomonas species): Bacterial soft rot is caused by several
species of bacteria, most commonly Erwinia carotovora. The bacteria colonize plants as a
consequence of wounding during cultivation, or from damage caused by insects or other
diseases. Symptoms initially appear as water soaked areas that turn soft, decay, liquefy, and
emit a distinctive foul odor. Hot wet weather promotes disease development. Burying crop
residues and rotating with a grain crop will reduce bacterial populations.
•
Downy mildew (Peronospora parasitica): Downy mildew is caused by a fungus that is
promoted by cool, wet weather in the spring and fall. Symptoms first appear as small
irregular yellow leaf spots on the upper leaf surface. A gray mold develops on the underside
of the leaves. During wet weather, the mold appears downy white. The vascular tissue may be
invaded and discolored, setting the stage for invasion by bacterial soft rot. The disease can
cause large losses in seedbeds.
•
White rot (Sclerotinia sclerotiorum): White rot (also called white mold or white blight) is a
soil-borne fungal disease. Symptoms first appear as water-soaked spots that enlarge into
long, irregular-shaped areas covered by white-cottony mold. The disease is typically first seen
on the stalk, base of petioles, or leaves closest to the ground. The lesions may girdle the
stalk, causing it to break. A diagnostic sign is the presence of elliptical black sclerotia (fungus
spore-bearing structures) which vary from 1/8” to ½” in length.
•
Black leg (Leptosphaeria maculans): Black leg is a fungal dry rot. Symptoms first appear as
black round spots. As the disease progresses, the spots enlarge becoming grayish-tan in color
punctuated with small black pycnidia (tiny black spore cases) which are diagnostic of the
disease. The stem may become constricted at the soil line.
•
Wire stem (Rhizoctonia solani): This is the fungus that commonly causes damping off of
seedlings and causes dry rot of the stem just below the soil line. It is common in most soils,
and is encouraged by damp conditions with poor air circulation.
•
Club root (Plasmodiophora brassicae): Caused by a fungus, club root causes leaf wilting and
yellowing, especially during especially during hot, sunny days. The diagnostic symptom is the
formation of spindle-like galls on roots. Germination of the spores of this soil-borne disease
is favored by a soil pH of 7.2 or less. Spores may persist in the soil for at least seven years,
and infected soil may be essentially unusable for brassica crops. Control can be achieved by
keeping the soil slightly alkaline.
Control measures for diseases:
Many disease problems (especially soil-borne diseases) can be prevented or managed by use of
disease-free seed and disease-free transplants, rotation of brassica crops with non-brassica crops for a
minimum of two to three years, burying or destroying infected crop residues, controlling insects that
damage plants, removing wild brassicas that serve as disease hosts, sanitation of tools and equipment
that have come into contact with diseased plants, and use of tolerant or resistant varieties.
Interestingly, there is a loose association between insect resistance and disease resistance in some
(but not all) varieties. This is no accident. The correlation is based on the fact that plants injured by
insects have opened the doors to opportunistic colonization by disease organisms. Also, certain
insect-resistant varieties are resistant by means of antibiosis, the production of phytochemicals that
in addition to having biological affects against insects may have biological effects on certain disease
organisms.
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Seed-borne diseases:
Organic growers can control seed-borne diseases by using hot water treatment of seed. A full
discussion of this technique is covered in the companion volume in this series: “Seed Processing and
Storage.” Other organic seed treatments are available at www.growseed.org/seedtreatments.html.
SELECTED BIBLIOGRAPHY AND LITERATURE CITED
Ashworth, Suzanne 1972. Seed to Seed: Seed Saving and Growing Techniques for Vegetable Gardeners.
Seed Savers Exchange, Decorah, IA.
Briggs, F.N. and P.F. Knowles. 1977. Introduction to Plant Breeding. University of California, Reinhold
Publishing Corporation, NY.
Deppe, Carol 2000. Breed Your Own Vegetable Varieties. Chelsea Green Publishing, White River
Junction, VT.
Free, John B. 1970. Insect Pollination of Crops. Academic Press, NY.
Haig, J.C. 1956. Plant breeding report. National Vegetable Research Station Report. 4: 10-13.
Hawthorn, Leslie R. and Leonard H. Pollard. 1954. Vegetable and Flower Seed Production. The
Blakiston Company, Garden City, NY
Lorenz, Oscar A. and Donald N. Maynard. 1980. Knott’s Handbook for Vegetable Growers (2nd. edition),
John Wiley & Sons, NY.
MacNab, A.A., Sherf, A.F., and J.K. Springer. 1983. Identifying Diseases of Vegetables. Pennsylvania
State University, University Park, PA.
McGregor, S. E. 1976. Insect Pollination of Cultivated Crop Plants. Agricultural Handbook No. 496, U.S.
Department of Agriculture. Washington, DC.
Schery, Robert W. 1972. Plants for Man. (2nd ed.). Prentice-Hall, Englewood Cliffs, NJ.
Yepson Jr., Roger B (ed.). 1976. Organic Plant Protection. Rodale Press, Emmaus, PA.
WORLD WIDE WEB RESOURCES
Due to the dynamic and ever-changing nature of resources available on the World Wide Web, the
following Internet addresses may not remain current. If the address has changed, try using the base
URL for locating information that has moved to a new address:
Mississippi State University: Commercial Production of Cabbage
http://msucares.com/pubs/infosheets/is1513.html
Ohio State University: Vegetable Seed Production – “Dry” Seeds
http://www.ag.ohio-state-edu/~seedsci/vsp02.html
Michigan State University: Vegetable Production and Management
http://www.rec.udel.edu/veggie/kee/colecrops.htm
CREDITS
Front matter illustration is from McGregor, 1976 (page 166).
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DISTRIBUTION AND LICENSING INFORMATION
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This is a summary of the Creative Commons Attribution-NonCommercial-NoDerivs License. To view
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